Posidonius (pōsēdō´nēəs), c.135–c.51 BC, Greek Stoic philosopher, b. Apamea, Syria. He settled in Rhodes after extensive travels. Noted for his learning, Posidonius gave new life to Stoicism by fortifying it with contemporary learning. Although his writings have been lost, it is known that they were copious. He made contributions to Stoic physics and ethics—notably the theory that a vital force emanating from the sun permeated the world and his doctrine of cosmic sympathy, through which man and all things in the universe are united. Other writings dealt with the natural sciences, mathematics, and military tactics. He had strong influence on the Romans.